1. Field of the Invention
The present invention relates to a process of coating reinforced thermoplastic substrates with a plurality of coatings to provide a coated substrate having improved visual and physical properties.
2. Brief Description of the Prior Art
The use of plastic components in combination with metal parts is common in the production of automobiles. Mixtures of resins an reinforcing materials such as fibers, flakes and particulate materials are used as bulk molding or sheet molding compounds, BMC and SMC, which are molded to form various shaped articles. Generally, in applications requiring high quality painted surfaces, highly filled thermosetting SMCs have been used.
In recent years thermoplastic resins which can be subjected to elevated temperatures have been developed. Due to their thermoplastic property, these materials are particularly attractive since trimmings and off-specification parts can be reclaimed for further use. However, the presence of reinforcing fibers and particulates adversely affects the appearance of coated articles unless the coated thermoplastic substrate is highly filled as with SMC substrates or the particles have small diameters and low aspect ratios.
When a reinforced thermoplastic substrate is coated and baked at an elevated temperature, the surface of the coated substrate appears to be smooth, however, upon cooling irregularities form on the surface giving an unacceptable distinctness of image (DOI). The development of surface irregularities appears to be associated with a difference in the coefficient of expansion between the thermoplastic resins and the reinforcing components. The irregularites are accented when the coated reinforced thermoplastic substrates are used in multi-component assemblies. The problem is particularly evident in applications in which reinforced thermoplastic panels are assembled adjacent metal panels in an automobile body. In the automobile industry any substantial difference in the quality of appearance of the coating on the plastic part and the adjacent metal part is unacceptable.
The quality of the surface of the coating on the parts can be measured using a commercially available distinctness of image (DOI) meter. Reflected light intensity from a photo detector is measured as a function of the scattering angle. If the scattering function is short and wide, the surface is said to have a low DOI. If the scattering is limited, the surface has high distinctness. DOI meters detect the reflected light in a region slightly away from the specular angle. If the light detected is small, the distinctness is high. DOI is measured on a scale from 0 to 100 with 100 being the highest level of smoothness.
Various methods have been disclosed for applying finishes to reinforced plastic substrates. U.S. Pat. No. 4,737,403 discloses coating a substrate with a specially formulated primer. From the DOI data disclosed in the patent, the use of the special primer appears to work well with an SMC "PHASE ALPHA" from Ashland Chemical Company. However, SMCs which are highly filled do not presemt the same problems as those encounted in coating reinforced thermoplastic materials which have a relatively low filler content.
Rendering the plastic substrate highly conductive so that the plastic part may be simulatenously electrocoated with metal parts by submerging the multi-material workpiece in an electrocoating bath is disclosed in U.S. Pat. No. 4,745,012. It is said that the primer applied to the plastic substrate renders the substrate coatable in the electrocoating bath. In coating the plastic substrate with the electrocoating primer, the primer is not utilized for its generally recognized purpose of providing corrosion protection of the metal parts. The quality of the final coated surface is not disclosed in the patent.
In practice, thermoplastic substrates having low reinforcing filler content require application of successive primer coatings and curing each primer coating before applying a topcoating in order to provide a quality coating with an acceptable DOI. Lightly sanding the primed surface improves the quality of the coating. Curing each primer coating entails heating the coated substrate to an elevated temperature for an extended period of time which is expensive from a time and cost standpoint.
It is therefore an object of this invention to provide a method for coating reinforced thermoplastic substrates which is cost efficient and provides products having improved properties.